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use num::Float;
use std::iter::{Product, Sum};
use std::slice::Iter;
#[allow(dead_code)]
pub fn present_value<T>(cash_flow: &T, period: usize, discount_rate: &T) -> T
where
T: Float + Product<T>,
{
let period: i32 = (period as i32) * -1;
let discount: T = T::one() + *discount_rate;
let discount_factor: T = discount.powi(period);
*cash_flow * discount_factor
}
#[cfg(test)]
mod present_value_tests {
use crate::present_value::present_value;
use num::abs;
#[test]
fn it_works_at_zero() {
let cash_flows: Vec<f32> = vec![0.0, 1.0, -1.0, 1234.56789, -1234.56789];
let period: usize = 0;
let discount_rate: f32 = 0.20;
for cash_flow in cash_flows {
assert_eq!(cash_flow, present_value(&cash_flow, period, &discount_rate));
}
}
#[test]
fn it_works_at_one() {
let cash_flows: Vec<f32> = vec![0.0, 1.0, -1.0, 1234.56789, -1234.56789];
let period: usize = 1;
let discount_rate: f32 = 0.20;
let expected_present_values: Vec<f32> = vec![0.00, 0.833, -0.833, 1028.806, -1028.806];
let precision: f32 = 0.001;
for index in 0..cash_flows.len() {
let actual_pv: f32 = present_value(&cash_flows[index], period, &discount_rate);
let expected_pv: f32 = expected_present_values[index];
assert!(abs(actual_pv - expected_pv) <= precision);
}
}
#[test]
fn it_works_at_two() {
let cash_flows: Vec<f32> = vec![0.0, 1.0, -1.0, 1234.56789, -1234.56789];
let period: usize = 2;
let discount_rate: f32 = 0.20;
let expected_present_values: Vec<f32> = vec![0.00, 0.6944, -0.6944, 857.338, -857.338];
let precision: f32 = 0.001;
for index in 0..cash_flows.len() {
let actual_pv: f32 = present_value(&cash_flows[index], period, &discount_rate);
let expected_pv: f32 = expected_present_values[index];
assert!(abs(actual_pv - expected_pv) <= precision);
}
}
}
#[allow(dead_code)]
pub fn from_cash_flows_and_discount_rate<T>(cash_flows: Iter<T>, discount_rate: &T) -> T
where
T: Float + Product<T> + Sum<T>,
{
cash_flows
.enumerate()
.map(|(period, cash_flow)| {
crate::present_value::present_value(cash_flow, period, discount_rate)
})
.sum()
}
#[cfg(test)]
mod from_cash_flows_and_discount_rate_tests {
use crate::present_value::from_cash_flows_and_discount_rate;
use num::abs;
#[test]
fn it_works_with_a_positive_npv() {
let cash_flows: Vec<f32> = vec![0.0, 1.0, -1.0, 1234.56789, -1234.56789];
let discount_rate: f32 = 0.20;
let precision: f32 = 0.001;
let expected_value: f32 = 119.2137;
let actual_value: f32 =
from_cash_flows_and_discount_rate(cash_flows.iter(), &discount_rate);
assert!(abs(expected_value - actual_value) <= precision);
}
#[test]
fn it_works_with_a_negative_npv() {
let cash_flows: Vec<f32> = vec![-500.0, 100.0, 2.0, 3.0, 4.0];
let discount_rate: f32 = 0.30;
let precision: f32 = 0.001;
let expected_value: f32 = -419.1275;
let actual_value: f32 =
from_cash_flows_and_discount_rate(cash_flows.iter(), &discount_rate);
assert!(abs(expected_value - actual_value) <= precision);
}
}